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Climates of Terrestrial Exoplanets and Biosignatures

Siddharth Bhatnagar, Emeline Bolmont, Nikita J. Boeren, Janina Hansen, Björn Konrad, Leander Schlarmann, Eleonora Alei, Marie Azevedo, Marrick Braam, Guillaume Chaverot, Jonathan Grone, Kaustubh Hakim, Mathilde Houelle, Daniel Kitzmann, Christophe Lovis, Antoine Pommerol, Sascha P. Quanz, Martin Turbet, Audrey Vorburger, Susanne F. Wampfler, Francis Zong Lang

TL;DR

This work addresses the challenge of understanding climates of terrestrial exoplanets and the detectability of biosignatures through an integrated, interdisciplinary framework. It presents a hierarchical modeling approach, including 1-D radiative-convective and energy balance models, and 3-D global climate models THOR and Generic-PCM, complemented by atmospheric retrieval techniques and mission-oriented studies such as LIFE. The contributions span model development, Solar System benchmarks, retrieval pipelines, and biosignature instrumentation concepts, demonstrating how climate physics, planetary interiors, and observational strategies coalesce to inform habitability assessments and biosignature detection. The findings underscore the sensitivity of Habitable Zone boundaries to atmospheric composition and pressure, the importance of ocean and cloud dynamics, and the potential of future facilities to detect Earth-like or novel biosignatures across nearby planetary systems, thereby shaping mission designs and data interpretation frameworks.

Abstract

Understanding the climates of terrestrial exoplanets and the detectability of biosignatures is an inherently interdisciplinary challenge, requiring the integration of insights from Solar System exploration, exoplanet observations and climate science. Building from Earth as the only known inhabited planet, NCCR PlanetS has developed models, tools and observational strategies to assess planetary environments far beyond direct reach. Between 2018 and 2025, PlanetS made major contributions across theory, modelling, instrumentation and mission preparation. On the modelling side, the Generic Planetary Climate Model enabled climate studies across a wide range of planetary regimes, from early Venus to temperate terrestrial exoplanets including Proxima b, incorporating advanced developments such as a dynamical slab ocean. In parallel, the THOR global climate model was developed to avoid Earth-centric assumptions and to stably simulate diverse atmospheric regimes. PlanetS has also advanced atmospheric retrieval techniques combining forward modelling, Bayesian inference and machine learning, applied to targets ranging from Solar System bodies to exoplanet phase curves and directly imaged spectra. These efforts have helped assess the scientific return of future missions, notably the Large Interferometer for Exoplanets (LIFE) and to define instrumental requirements for detecting Earth-like atmospheres and biosignatures. Within the Solar System, PlanetS contributed key technologies for biosignature detection, including ORIGIN and SenseLife, enabling in-situ and remote detection of organics, isotopic ratios and microstructures. Finally, PlanetS has played a major role in preparing the next generation of observatories, from JWST, VLT and ELT instruments to LIFE and the Habitable Worlds Observatory. Together, these contributions form an integrated framework advancing the search for life beyond Earth.

Climates of Terrestrial Exoplanets and Biosignatures

TL;DR

This work addresses the challenge of understanding climates of terrestrial exoplanets and the detectability of biosignatures through an integrated, interdisciplinary framework. It presents a hierarchical modeling approach, including 1-D radiative-convective and energy balance models, and 3-D global climate models THOR and Generic-PCM, complemented by atmospheric retrieval techniques and mission-oriented studies such as LIFE. The contributions span model development, Solar System benchmarks, retrieval pipelines, and biosignature instrumentation concepts, demonstrating how climate physics, planetary interiors, and observational strategies coalesce to inform habitability assessments and biosignature detection. The findings underscore the sensitivity of Habitable Zone boundaries to atmospheric composition and pressure, the importance of ocean and cloud dynamics, and the potential of future facilities to detect Earth-like or novel biosignatures across nearby planetary systems, thereby shaping mission designs and data interpretation frameworks.

Abstract

Understanding the climates of terrestrial exoplanets and the detectability of biosignatures is an inherently interdisciplinary challenge, requiring the integration of insights from Solar System exploration, exoplanet observations and climate science. Building from Earth as the only known inhabited planet, NCCR PlanetS has developed models, tools and observational strategies to assess planetary environments far beyond direct reach. Between 2018 and 2025, PlanetS made major contributions across theory, modelling, instrumentation and mission preparation. On the modelling side, the Generic Planetary Climate Model enabled climate studies across a wide range of planetary regimes, from early Venus to temperate terrestrial exoplanets including Proxima b, incorporating advanced developments such as a dynamical slab ocean. In parallel, the THOR global climate model was developed to avoid Earth-centric assumptions and to stably simulate diverse atmospheric regimes. PlanetS has also advanced atmospheric retrieval techniques combining forward modelling, Bayesian inference and machine learning, applied to targets ranging from Solar System bodies to exoplanet phase curves and directly imaged spectra. These efforts have helped assess the scientific return of future missions, notably the Large Interferometer for Exoplanets (LIFE) and to define instrumental requirements for detecting Earth-like atmospheres and biosignatures. Within the Solar System, PlanetS contributed key technologies for biosignature detection, including ORIGIN and SenseLife, enabling in-situ and remote detection of organics, isotopic ratios and microstructures. Finally, PlanetS has played a major role in preparing the next generation of observatories, from JWST, VLT and ELT instruments to LIFE and the Habitable Worlds Observatory. Together, these contributions form an integrated framework advancing the search for life beyond Earth.
Paper Structure (35 sections, 6 figures)

This paper contains 35 sections, 6 figures.

Table of Contents

  1. Introduction
  2. Climate Simulations and Habitability
  3. Tools to study habitability
  4. 1-D radiative-convective models
  5. 1-D Energy Balance Models
  6. 3-D Global Climate Models
  7. Atmospheric retrievals
  8. Habitability in the solar system
  9. Earth
  10. Early and modern Mars
  11. Venus
  12. Ocean worlds and moons
  13. Habitability of exoplanets
  14. Concept of the habitable zone
  15. The inner edge of the HZ
  16. ...and 20 more sections

Figures (6)

  • Figure 1: The different Solar System bodies and exoplanets this chapter focuses on, and associated missions. Those with a strong PlanetS involvement are shown with an asterisk.
  • Figure 2: The various factors affecting planetary evolution and habitability. Font colors indicate observability: blue for directly detectable features with sufficiently powerful telescopes, green for those requiring model-based interpretation and orange for properties accessible mainly through theory. Credits: 2018haex.bookE..57M
  • Figure 3: The various climate models used in the PlanetS community.
  • Figure 4: Different limits of the Habitable Zone for different (1-D and 3-D) models, different assumptions for the spin of the planet, composition of the atmosphere and its pressure. Most inner edge limits are given for the runaway greenhouse transition, but the red limit is given for the water condensation limit of Turbet:2023. This figure is inspired from Figure 8 of Turbet:2023 with additions from more recent works like 2023AA...680A.103C and 2025Life...15...79K.
  • Figure 5: A Illustration of the principle of induced circular polarization. Chiral macromolecules can change the polarization of scattered light to a fraction of circular polarization. B Examples of circular polarization spectra. Upper panel: a leaf in reflectance. Lower panel: a Synechococcus (cyanobacteria) culture in transmittance. Pigment types can be recognized as circular polarization peaks in the respective absorption bands. Figure from 2025arXiv250703819B.
  • ...and 1 more figures